Letter 121 — On Instinct in Animals (§8)

“Thus the child who is trying to stand and is becoming used to carry his own weight, on beginning to test his strength, falls and rises again and again with tears until through painful effort he has trained himself to the demands of nature. And certain animals with hard shells, when turned on their backs, twist and grope with their feet and make motions side-ways until they are restored to their proper position. The tortoise on his back feels no suffering; but he is restless because he misses his natural condition, and does not cease to shake himself about until he stands once more upon his feet.”